I want to play raw pcm (signed 8 bit) that is stored in a byte array in my C#.NET application.
I looked through the examples for VC# that came with the API package but it only showed how to play files out of memory containing all header structure and so on but I just have not more than raw data and a samplerate stored in a variable.

It seems like you will have to use a combination of FMOD_OPENMEMORY, FMOD_CREATESAMPLE and, maybe, FMOD_OPENRAW when creating the sound, plus some FMOD_CREATESOUNDEXINFO. The OPENMEMORY explains itself, the CREATESAMPLE will copy the data to fmod internal buffer so you can free your buffer after creating the sound (mixing native and managed code is a pain and this should minimize the need for that). OPENRAW "will ignore file format and treat as raw pcm" as the documentation states, but I don’t know if you will need it.